1080p on a 4K Display

So I've been working on laptops and other small screens for the last few years and neglected my big screen experience. I now want to get back to having a bigger screen but I was just wondering about something.

What will 1080p content look like on a 4k screen. I'm pretty tempted to get Asus's new 4K panel, I'm building a new PC for it's day to day, but I was wondering what would happen if I plugged in say a PS3 or XBOX360 which obviously don't punch out at that high resolution. Does anyone know it would behave?